Iii-v photovoltaic elements

ABSTRACT

Solar cell structures that have improved carrier collection efficiencies at a heterointerface are provided by low temperature epitaxial growth of silicon on a III-V base. Additionally, a solar cell structure having improved open circuit voltage includes a shallow junction III-V emitter formed by epitaxy or diffusion followed by the epitaxy of Si x Ge 1−x  passivated by amorphous Si y Ge 1−y :H.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This patent application is a divisional of U.S. patent application Ser.No. 14/500,538 filed Sep. 29, 2014, entitled “III-V PHOTOVOLTAICELEMENTS,” which is in turn a divisional of U.S. patent application Ser.No. 13/193,260 filed Jul. 28, 2011, entitled “III-V PHOTOVOLTAICELEMENTS.” The complete disclosures of the aforementioned U.S. patentapplication Ser. Nos. 13/193,260 and 14/500,538 are expresslyincorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to the physical sciences, and, moreparticularly, to solar cell structures comprising III-V absorbermaterials.

BACKGROUND OF THE INVENTION

Direct gap III-V materials such as gallium arsenide are attractivecandidates for making high efficiency solar cells due to their strongabsorption properties. The fabrication of high efficiency III-V solarcells can be achieved by epitaxial growth of the structures usingvarious techniques such as metalorganic chemical vapor deposition andmolecular beam epitaxy, the cost of which renders them relativelyexpensive for many terrestrial applications. Alternative approaches havebeen proposed such as formation of ITO Schottky contacts and zincdiffusion. Such approaches do not, however, provide a platform formanufacturing high efficiency cells due to the lack of proper surfacepassivation.

SUMMARY OF THE INVENTION

Principles of the invention provide techniques for improving solar cellperformance and for solar cell structures that exhibit improved carriercollection efficiency at the heterointerface. In one aspect, anexemplary method includes the steps of obtaining a doped, p-type III-Vbase layer, depositing an n+ amorphous hydrogenated silicon layer or n+epitaxial crystalline Si on the base layer, and annealing the sample,thereby forming an emitter layer comprising an n+ region in the baselayer and an n+ epitaxial layer adjoining the emitter layer. Furthermanufacturing steps may include forming an intrinsic amorphoushydrogenated semiconductor layer on the n+ epitaxial layer and atransparent conductive layer such as a TCO over the intrinsic amorphoushydrogenated semiconductor layer.

A further exemplary method includes obtaining a doped, III-V base layerand forming an intrinsic hydrogenated epitaxial semiconductor layercomprising Si_(x)Ge_(1−x) wherein x is between 0 and 1 on the base layerby plasma enhanced chemical vapor deposition at a temperature below 400°C. An intrinsic amorphous semiconductor layer comprisingSi_(x′)Ge_(1−x′):H, wherein x′ is between 0 and 1 is formed on theintrinsic epitaxial semiconductor layer and an emitter layer is formedon the intrinsic amorphous semiconductor layer.

Photovoltaic elements are provided in accordance with further exemplaryembodiments of the invention. In one such embodiment, a solar cellstructure is provided that comprises a doped III-V absorber layer, anepitaxial, intrinsic semiconductor layer adjoining the absorber layer,an emitter layer above the epitaxial, intrinsic semiconductor layer, anda transparent conductive layer above the emitter layer. The intrinsic,epitaxial semiconductor layer may comprise Si_(x)Ge_(1−x) wherein x isbetween 0 and 1. An intrinsic amorphous semiconductor layer preferablyadjoins the intrinsic, epitaxial semiconductor layer and may becomprised of Si_(x′)Ge_(1−x′):H, wherein x′ is between 0 and 1. Theemitter layer may be comprised of Si_(y)Ge_(1−y):H, wherein y is between0 and 1.

In accordance with a further exemplary embodiment, a solar cellstructure comprises a p-type III-V base layer and an n+ III-V emitterlayer adjoining the base layer. An n+ epitaxial layer adjoins theemitter layer, the n+ epitaxial layer comprising Si_(x)Ge_(1−x) whereinx is between 0 and 1. An intrinsic amorphous semiconductor layercomprised of Si_(y)Ge_(1−y):H wherein y is between 0 and 1 adjoins then+ epitaxial layer and a transparent conductive layer is above theintrinsic amorphous semiconductor layer.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

Techniques of the present invention can provide substantial beneficialtechnical effects, including methods of making high efficiency solarcells having particular utility in solar panels. The resultingstructures can, however, be used for energy autonomous solar-poweredelectronic circuits and sensors. One or more embodiments may provide oneor more of the following advantages:

-   -   Improving open circuit voltage (V_(oc))    -   Improving the short circuit current density (J_(sc))    -   Improving solar cell efficiency (η)    -   Improving the fill factor (FF)

These and other features and advantages of the present invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a chart describing three exemplary single heterojunctionsolar cell structures;

FIG. 2 includes a graph illustrating various properties of the exemplarystructures shown in FIG. 1;

FIG. 3 is a cross sectional view of one the exemplary structures shownin FIG. 1 using a transmission electron microscope (TEM);

FIG. 4 is a schematic illustration of a solar cell structure includingan intrinsic, amorphous semiconductor layer on a III-V base;

FIG. 5 is a schematic illustration of a solar cell structure inaccordance with an embodiment of the invention;

FIG. 6 is a schematic illustration of a solar cell structure inaccordance with a further embodiment of the invention,

FIGS. 7A-7C illustrate a process for forming an exemplary solar cellstructure in accordance with an embodiment of the invention, and

FIG. 8 is an energy band diagram relating to the solar cell structureshown in FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Heterojunction III-V solar cell structures based on a-Si:H/III-Vheterostructures offer a path for low-cost, high efficiency PV(photovoltaic) technology when implemented in conjunction with a layertransfer technique. The use of a-Si:H as an intrinsic layer (i-a:Si:H)can significantly improve the surface passivation of GaAs. US Pub. No.2010/0307572 entitled “Heterojunction III-V Photovoltaic CellFabrication”, the disclosure of which is incorporated by referenceherein, discloses techniques for forming single and doubleheterojunction III-V PV cells.

FIG. 4 shows a solar cell structure 10 that can be produced usingtechniques disclosed in US Pub. No. 2010/0307572. The structure 10includes a base layer 12 comprising a doped III-V substrate that may ben-type or p-type. Doped amorphous silicon layers 18, 18′ are formed onintrinsic amorphous silicon layers 16 on the base layer 12. Theamorphous silicon layers 18, 18′ may have doping types that are the sameor opposite from the doping type of the base layer. If the doping typeof an amorphous silicon layer is opposite to that of the base layer, theamorphous silicon layer will function as an emitter. In the structureshown in FIG. 4, one of the amorphous silicon layers 18′ has the samedoping type as the base layer while the other amorphous silicon layerhas the opposite doping type. Transparent layers 20 such as atransparent conducting oxide (TCO) are formed on the amorphous siliconlayers 18. Front metal contacts including fingers and bus bars (notshown) may be formed on the transparent layers. Although the i-a:Si:Hlayers 16 improve surface passivation of the GaAS base layer, therelatively high interface trap density at the i-a:Si:H/GaAsheterointerface leads to a significantly high dark current, degradingthe open circuit voltage.

FIG. 1 includes a chart relating to three structures, one of which issimilar to that shown in FIG. 4 in that an intrinsic amorphoussemiconductor layer adjoins a III-V base and a doped hydrogenatedamorphous silicon layer is formed over the intrinsic semiconductorlayer. The intrinsic semiconductor layer of this “Sample 1” or“Reference Structure” has a relatively low hydrogen dilution (HD) ratio,resulting in an amorphous structure. The structure includes an InGaPback surface field (BSF). Specific thicknesses are provided for thelayers of the “Reference Structure” of FIG. 1 to facilitate performancecomparisons to the other two structures, each of which has correspondinglayers of equal thickness. Those of ordinary skill in the art willappreciate that the layers may be provided in different thicknesses.

The structures identified in FIG. 1 as “New Structure 1” and “NewStructure 2” are examples of single heterojunction structures that maybe provided in accordance with the invention. It will, however, beappreciated that the principles of the invention can also be applied todouble heterojunction solar cell structures. Both structures includeintrinsic semiconductor layers adjoining a GaAs absorber. Theseintrinsic semiconductor layers are epitaxial and were grown atrelatively high hydrogen dilution ratios at below 200° C. using plasmaenhanced chemical vapor deposition (PECVD). New Structure 2 furtherincludes an amorphous hydrogenated intrinsic semiconductor layer havinga relatively low hydrogen dilution ratio that overlies the epitaxiallayer. The variations in the hydrogen dilution ratios of the epitaxialand amorphous i-Si:H layers is achieved by varying the hydrogen gas flowwhile flowing the silicon precursor (silane (SiH₄) in this illustrativeembodiment) at a fixed rate. The low temperature epitaxial growth of Sicontaining carbon, germanium or other elements is discussed further inU.S. application Ser. No. 13/032,866 filed Feb. 23, 2011 and entitledLOW-TEMPERATURE SELECTIVE EPITAXIAL GROWTH OF SILICON FOR DEVICEINTEGRATION, the disclosure of which is incorporated by referenceherein.

FIG. 2 provides various performance parameters for the three structuresdescribed in FIG. 1. The graph shows the short circuit current densityas a function of voltage. The open circuit voltage, short circuitcurrent density, fill factor and efficiency of the three structures areprovided in chart form. While V_(oc) for Sample 1 (the “ReferenceStructure”) is greater than in Samples 2 and 3, the short circuitcurrent density, fill factor and efficiency of Samples 2 and 3 exceedthat of Sample 1.

A TEM image of Sample 2 (New Structure 1) is provided in FIG. 3. Thep-GaAS absorber layer, the epitaxial instrinsic semiconductor layer, theemitter layer (n+ a-Si:H) and a TCO layer comprised of ZnO:Al aredelineated in this figure. The BSF does not appear in the image. In thisembodiment, the epitaxial hydrogenated intrinsic silicon layer is grownat 150° C.

It will be appreciated that the III-V absorber layer of New Structure 1or New Structure 2 could be In_(x)Ga_(1−x)As with x ranging from 0 to0.53. The back contact employed can be selected from conventional backsurface fields known to those of ordinary skill in the art.

FIG. 5 shows a solar cell structure 50 in accordance with a furtherembodiment of the invention. Certain layers of the structure 50 alsofound in the solar cell structure 10 shown in FIG. 4 are designated withthe same reference numerals. The solar cell structure 50 includes adoped III-V base 12 such as GaAs or In_(x)Ga_(1−x)As where x can rangefrom 0-0.53. The absorbing material comprising the base canalternatively be In_(y)Ga_(1−y)P where y ranges from 0.4 to 1. The basedoping level is from 1e14-1e18/cm³. Intrinsic semiconductor layers 52comprised of epitaxial Si_(x)Ge_(1−x) adjoin the base. These layers 52contain hydrogen if grown using PECVD. The value of x may be between 0and 1, preferably 0-0.5, for these layers 52. The deposition temperatureof these epitaxial layers 52 is between 150-400° using PECVD. Thepreferred thickness of the epitaxial layers is 3-15 nm. Epitaxial SiGelayers 52 containing less than 35% Ge can be grown using conventionalCVD at temperatures exceeding 600° C. The layers 52 do not containhydrogen if grown using CVD. Low temperature PECVD growth of theepitaxial layers is preferred to allow low temperature fabrication ofheterojunction III-V solar cells and is particularly important forapplications such as flexible solar cells.

Intrinsic hydrogenated amorphous semiconductor layers 54 comprised ofSi_(x′)Ge_(1−x′):H adjoin the intrinsic epi-Si_(x)Ge_(1−x) layers inthis preferred embodiment, where x′ is between 0 and 1 and preferablybetween 0 and 0.5. These layers can be deposited using PECVD or hot-wirechemical vapor deposition including appropriate source materials. In oneembodiment, the intrinsic hydrogenated amorphous semiconductor layers 54are deposited in a process chamber containing a semiconductor precursorsource gas and a carrier gas including hydrogen. Hydrogen atoms withinthe carrier gas are incorporated into the deposited material to form theintrinsic hydrogenated semiconductor containing material of theintrinsic semiconductor layer. The germanium content in the hydrogenatedamorphous SiGe layers 54 may be different from the epitaxial layers 52.Carbon atoms can be incorporated in the amorphous semiconductor layerswith or in place of germanium. The germanium and carbon content can begraded. The amorphous semiconductor layers can each be comprised ofmulti-layer structures with different compositions. If carbon is used inplace of germanium in the above-referenced formula, the value of x′would be 0 to 0.6 and preferably 0 to 0.3.

A doped hydrogenated amorphous emitter layer 56 adjoins one of the twoamorphous semiconductor layers. Its doping type is opposite to that ofthe base 12. This layer 56 is comprised of a-Si_(y)Ge_(1−y):H where y isbetween 0 and 1 and preferably between 0 and 0.5. A back surface fieldlayer 56′ adjoins the other of the two intrinsic hydrogenated amorphoussemiconductor layers 54. Its doping type is the same as the doping typeof the base 12. This layer is comprised of a-Si_(z)Ge_(1−z):H where z isbetween 0 and 1 and preferably between 0 and 0.5.

Transparent conductive layers 20 adjoin the emitter and back surfacefield layers 56,56′. The transparent layers are comprised of conductivematerial that is transparent in the range of electromagnetic radiationat which photogeneration of electrons and holes occur within the solarcell structure 50. The transparent conductive layer 20 in the exemplaryembodiment of FIG. 5 may comprise TCO's such as indium tin oxide (ITO),tin oxide (SnO), fluorine-doped tin oxide (SnO₂:F) or aluminum-dopedzinc oxide (ZnO:Al). Transparent conducting films such as carbonnanotube-based films and graphene-based films may alternatively beemployed to form this transparent conductive layer 20. These examplesare to be considered exemplary as opposed to limiting. The thickness ofthe transparent conductive layer may vary depending on the type oftransparent conductive material employed as well as the technique usedin forming this layer. Typically, and in one exemplary embodiment, thethickness of the transparent conductive layer 20 is between 20-500 nm.Other thicknesses, including those less than 20 nm and/or greater than500 nm, can alternatively be employed. The preferred thickness of thetransparent conductive layer for minimizing reflection from the surfaceof Si is in the range of 70-110 nm for a TCO. Nanotube andgraphene-based films may in the range of 2-50 nm. The transparentconductive layer is typically formed using a deposition process such assputtering or CVD. Examples of CVD processes usable for a number oftypes of such layers include APCVD, LPCVD, PECVD, MOCVD and combinationsthereof. RF and DC magnetron sputtering are among other techniques forforming the transparent conductive layer 20.

The hydrogenated amorphous SiGe layers 56,56′ may be formed fromprecursor gases such as SiH₄, SiF₄, or H₂SiCl₂ (DCS). Germane is a knownprecursor for forming layers comprising germanium. The layers may bedoped “in situ” by adding a dopant gas containing dopant atoms in thegas mixture. The dopant atoms are incorporated into the depositedmaterial to form a hydrogenated doped semiconductor. Examples of dopantgases containing p-type dopant atoms are B₂H₆ and B(CH₃)₃ (TMB).Examples of an n-type dopant gas include AsH₃ and PH₃. The n-type dopantconcentration in certain layer(s) of the structure 50 ranges from 10¹⁶atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸-10²⁰ atoms/cm³ beinga typical range. The doping efficiency (the ratio of activated dopantatoms to the total dopant atoms) typically ranges from 0.1%-20%,although higher and lower doping efficiencies are possible. The dopingefficiency is generally decreased by increasing the dopant atomconcentration. The p-type dopant concentration likewise ranges from10¹⁶-10²¹ atoms/cm³ with the range of 10¹⁸-10²⁰ atoms/cm³ being typical.In this exemplary embodiment, the layer 56 above the base layer 12functions as an emitter and the layer 56′ below the base layer as a backsurface field (BSF).

FIG. 6 schematically illustrates a solar cell structure 60 in accordancewith a further exemplary embodiment of the invention. This embodimentincludes a p-type III-V base layer 12. A high interface state or trapdensity (D_(A)) limits performance of a solar cell structure by reducingV_(oc). The solar cell structure 60 helps to overcome such performancelimits. A shallow junction n+ III-V emitter layer 62 can be formed onthe base layer through epitaxial growth or introduction of n-type dopingimpurities into the base layer 12 through ion implantation or diffusion.Epitaxial methods include chemical vapor deposition such as MOCVD andmolecular beam epitaxy (MBE). The second possible method includes theintroduction of n-type doping impurities such as Si, Ge, Te or S intothe p-type base layer 12 via ion implantation and activation annealingat elevated temperatures in the range of 450-1000° C. Alternatively, theimpurities can be diffused out of an impurity-rich layer into the p-typebase layer. The impurity-rich layer could be, for example,a-Si_(1−x)Ge_(x) or a thin sulfur layer. The diffusion temperaturevaries depending on the type of the impurity and ranges between500-1000° C. The thickness of the n+ emitter layer 62 formed viaepitaxy, ion implantation or diffusion ranges from 5-50 nm. Dopingconcentrations may be in the same ranges as in the embodiment of FIG. 5.

An n+ epi-Si_(x)Ge_(1−x) layer 64 is formed on the emitter layer 62where x is between 0 and 1 and preferably between 0 and 0.5. This layermay, but does not necessarily contain hydrogen, the presence of hydrogendepending on growth conditions. The conduction band edge of the III-Vbase 12 (GaAs in this exemplary embodiment) and the n+epi-Si_(x)Ge_(1−x) layer 64 are aligned. In other words, the energydifference between the conduction band and vacuum is about the same forboth regions. (See FIG. 8.) V_(oc) degradation is thereby reduced. Anintrinsic amorphous hydrogenated semiconductor layer 66 adjoins the n+epi-Si_(x)Ge_(1−x) layer 64. The i-a:Si_(y)Ge_(1−y):H layer 66 (where yis between 0 and 1, preferably between 0 and 0.5) provides surfacepassivation of the adjoining n+ epi-Si_(x)Ge_(1−x) layer. The remaininglayers of the solar cell structure 60 in this embodiment are the same asthose found in the structure 50 according to the embodiment of FIG. 5and have been designated with the same reference numbers in thedrawings.

FIGS. 7A-7C show several steps employed in the manufacture of anexemplary solar cell structure having a p-type III-V base layer and anovel emitter layer in accordance with the invention. It will beappreciated that the structure may include an appropriate back surfacefield region at the rear section of the structure.

Referring to FIG. 7A, a p-type GaAs base layer 12 is formed on a p+ GaAssubstrate 70. The front contact (emitter layer) for the structure isformed by depositing a 5-20 nm thick n+ a-Si:H layer 72 as an impurityrich material on the base layer. In this exemplary embodiment, the n+a-Si:H layer 72 is phosphorus doped, but could alternatively be dopedwith arsenic. The structure shown in FIG. 7A is subjected to rapidthermal annealing (RTA) at elevated temperatures as shown in FIG. 7B. Inthis exemplary embodiment, the annealing temperature is 900° C. and theannealing time is 60-240 seconds. Upon annealing, some silicon atomsdiffuse into the GaAs base to form an n+ region 74 about 10-15 nm deepwhile the a-Si:H layer becomes single-crystalline through a solid-phaseepitaxy process, forming the n+ epi-Si layer 76 shown in FIG. 7B. Thislayer 76 is unlikely to contain hydrogen as Si:H bonds are volatileabove 400-500° C. An intrinsic semiconductor layer (i-a-Si:H) 78 isformed on the n+ epi-Si layer 76 and a transparent conductive layer 20is formed on the intrinsic amorphous semiconductor layer 78 to providethe structure 80 shown in FIG. 7C. In this exemplary embodiment, thetransparent conductive layer 20 is a TCO such as ZnO:Al. V_(oc) of theresulting structure 80 is 920 mV.

The method as described with respect to FIGS. 7A-7C can be effected bydepositing an n+ epitaxial crystalline layer on the p-type base layerrather than the n+ a-Si:H layer, followed by annealing of the sample.

Given the discussion thus far, it will be appreciated that, in generalterms, an exemplary method, according to an aspect of the invention,includes the step of obtaining a doped, p-type III-V base layer such asthat shown in FIG. 7A. An n+ amorphous hydrogenated silicon layer isdeposited on the base layer, also as shown in FIG. 7A. The base layerand the n+ amorphous hydrogenated silicon layer are annealed, preferablyby rapid thermal annealing, forming an emitter layer 74 comprising an n+region in the base layer and an n+ epitaxial layer 76 adjoining theemitter layer as shown in FIG. 7B. Further manufacturing steps mayinclude forming an intrinsic amorphous hydrogenated semiconductor layer78 on the n+ epitaxial layer and a transparent conductive layer 20 suchas a TCO over the intrinsic amorphous hydrogenated semiconductor layerto provide the photovoltaic element shown in FIG. 7C.

A further exemplary method in accordance with the invention may beemployed to form at least part of the solar cell structure 50 shown inFIG. 5. The exemplary method includes obtaining a doped, III-V baselayer and forming an intrinsic hydrogenated epitaxial semiconductorlayer comprising Si_(x)Ge_(1−x) wherein x is between 0 and 1 on the baselayer by plasma enhanced chemical vapor deposition at a temperaturebelow 400° C. An intrinsic amorphous semiconductor layer comprisingSi_(x′)Ge_(1−x′):H, wherein x′ is between 0 and 1 is formed on theintrinsic epitaxial semiconductor layer and an emitter layer is formedon the intrinsic amorphous semiconductor layer. As discussed above, themethod may further include forming a transparent conductive layer on theemitter layer. The method may include further steps depending on whethera single or double heterojunction solar cell structure is desired.

In accordance with a further aspect of the invention, a solar cellstructure is provided that comprises a doped III-V absorber layer suchas the GaAs absorber described with respect to FIG. 1. An epitaxial,intrinsic semiconductor layer comprising hydrogenated silicon adjoinsthe absorber layer as described above with respect to New Structures 1and 2 of FIG. 1. An emitter layer is above the epitaxial, intrinsicsemiconductor layer and a transparent conductive layer is above theemitter layer as shown in FIG. 3.

A solar cell structure provided in accordance with another aspect of theinvention comprises a doped III-V base layer and an intrinsichydrogenated epitaxial semiconductor layer comprising Si_(x)Ge_(1−x)wherein x is between 0 and 1, the intrinsic hydrogenated epitaxialsemiconductor layer adjoining the base layer. FIG. 5 provides anexemplary embodiment of a structure 50 having such a base layer 12 andepitaxial layer 52. An intrinsic amorphous semiconductor layer adjoinsthe intrinsic hydrogenated epitaxial semiconductor layer and comprisedof Si_(x′)Ge_(1−x′):H, wherein x′ is between 0 and 1. Layer 54 in thestructure 50 shown in FIG. 5 is exemplary of such a layer. A dopedamorphous emitter layer comprised of Si_(y)Ge_(1−y):H, wherein y isbetween 0 and 1, is located above the intrinsic amorphous semiconductorlayer and a transparent conductive layer is positioned above the dopedamorphous emitter layer. Layers 56 and 20 in FIG. 5, respectively,illustrate corresponding structures. A structure including theabove-referenced layers can be incorporated in either a single or doubleheterojunction solar cell structure.

A solar cell structure in accordance with a further aspect of theinvention includes a p-type III-V base layer, an n+ III-V emitter layeradjoining the base layer, and an n+epitaxial layer adjoining the emitterlayer. The n+ epitaxial layer comprises Si_(x)Ge_(1−x) wherein x isbetween 0 and 1. An intrinsic amorphous semiconductor layer adjoins then+ epitaxial layer and is comprised of Si_(y)Ge_(1−y):H wherein y isbetween 0 and 1. A transparent conductive layer is located above theintrinsic amorphous semiconductor layer. FIG. 6 shows such a structure.In the embodiment of FIG. 6, the structure further includes an intrinsicepitaxial semiconductor layer 52 comprising Si_(x)Ge_(1−x) adjoining thebase layer, an intrinsic amorphous semiconductor layer 54 comprisingSi_(x′)Ge_(1−x′):H, an amorphous back surface field layer 56′ comprisingSi_(z)Ge_(1−z):H below the intrinsic amorphous semiconductor layer(wherein z is between 0 and 1), and a transparent conductive layer 20 (aTCO in this exemplary embodiment) below the amorphous back surface fieldlayer. FIG. 7C shows another exemplary embodiment of such a structurewherein the base and emitter layers 12, 74 both comprise GaAs and theintrinsic amorphous semiconductor layer 78 is i-a:Si:H.

Those skilled in the art will appreciate that the exemplary structuresdiscussed above can be distributed in raw form or incorporated as partsof intermediate products or end products that benefit from havingphotovoltaic elements therein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Terms such as “above” and “below” areused to indicate relative positioning of elements or structures to eachother as opposed to relative elevation.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A solar cell structure comprising: a doped III-Vabsorber layer; an epitaxial, intrinsic semiconductor layer adjoiningthe absorber layer; an emitter layer above the epitaxial, intrinsicsemiconductor layer, and a transparent conductive layer above theemitter layer.
 2. The solar cell structure of claim 1, furthercomprising an amorphous, intrinsic semiconductor layer comprisinghydrogenated silicon adjoining the epitaxial, intrinsic semiconductorlayer.
 3. The solar cell structure of claim 1, wherein the emitter layercomprises n+ a-Si:H and the absorber layer is a p-type layer.
 4. Thesolar cell structure of claim 1, wherein the epitaxial, intrinsicsemiconductor layer is hydrogenated, and further comprising anamorphous, intrinsic semiconductor layer comprising hydrogenated siliconadjoining the epitaxial, hydrogenated intrinsic semiconductor layer. 5.The solar cell structure of claim 1, wherein the epitaxial, intrinsicsemiconductor layer comprises Si_(x)Ge_(1−x):H wherein x is between 0and 1, and further including an intrinsic amorphous semiconductor layeradjoining the intrinsic epitaxial semiconductor layer and comprised ofSi_(x′)Ge_(1−x′):H, wherein x′ is between 0 and
 1. 6. The solar cellsstructure of claim 5, wherein the emitter layer is comprised of doped,amorphous Si_(y)Ge_(1−y):H, wherein y is between 0 and
 1. 7. The solarcell structure of claim 6, wherein x is between 0 and 0.5.
 8. The solarcell structure of claim 7, wherein x′ is between 0 and 0.5.
 9. The solarcell structure of claim 6, wherein y is between 0 and 0.5.
 10. The solarcell structure of claim 5, wherein the base layer comprises GaAs. 11.The solar cell structure of claim 5, further including a back surfacefield below the base layer.
 12. A solar cell structure comprising: ap-type III-V base layer; an n+ III-V emitter layer adjoining the baselayer; an n+ epitaxial layer adjoining the emitter layer, the n+epitaxial layer comprising Si_(x)Ge_(1−x) wherein x is between 0 and 1;an intrinsic amorphous semiconductor layer adjoining the n+ epitaxiallayer and comprised of Si_(y)Ge_(1−y):H wherein y is between 0 and 1,and a transparent conductive layer above the intrinsic amorphoussemiconductor layer.
 13. The solar cell structure of claim 12, whereinthe base layer and the emitter layer comprise GaAs.
 14. The solar cellstructure of claim 12, wherein x is between 0 and 0.5.
 15. The solarcell structure of claim 14, wherein y is between 0 and 0.5.
 16. Thesolar cell structure of claim 12, further including: an intrinsicepitaxial semiconductor layer comprising Si_(x)Ge_(1−x) adjoining thebase layer; a second intrinsic amorphous semiconductor layer comprisingSi_(x′)Ge_(1−x′):H wherein x′ is between 0 and 1, the second intrinsicamorphous semiconductor layer adjoining the intrinsic epitaxialsemiconductor layer; an amorphous back surface field layer comprisingSi_(z)Ge_(1−z):H below the intrinsic amorphous semiconductor layerwherein z is between 0 and 1, and a transparent conductive layer belowthe amorphous back surface field layer.
 17. The solar cell structure ofclaim 16, wherein x, x′, y and z are between 0 and 0.5.
 18. The solarcell structure of claim 16, wherein the base layer and the emitter layercomprise GaAs.